pH Concept

Chapter 16

Self-Ionization of Water

Write a reaction showing water interacting with water.H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

If the reaction reaches equilibrium, then a constant ratio of products to reactants exists. The constant ratio can be calculated in the lab by measuring the

concentration of each substance in the reaction and setting up an expression. The expression are called “k” expressions.

Since only solutions (NOT pure liquids, solids, or gases) have concentrations, the expression should only consider aqueous solutions.

The expression for this equation is: k = [H3O+] [OH-] After many lab tests to confirm, the Kw value = 1.0x10-14.

This means that water reacting with water will always produce a ratio of ions equal to 1.0x10-14.

Applications of Kw

Water is the solvent in all acid and base dissociation reactions.

In a beaker full of pure water:H2O(l) + H2O(l) H3O+(aq) + OH-(aq)

Kw = [H3O+] [OH-] = 1.0x10-14

Look closely at the K expression. Make a prediction. Which ion will increase if an ACID

is dissolved in the water? How will the concentration of the other ion respond?

Make a prediction. Which ion will increase if a BASE is dissolved? How will the concentration of the other ion respond?

Which ion will increase if an BASE is dissolved in the water? How will the

concentration of the other ion respond?

A.) H3O+ will increase. OH- will increase.

B.) H3O+ will increase. OH- will decrease.

C.) H3O+ will decrease. OH- will increase.

D.) H3O+ will decrease. OH- will decrease.

Analysis Level Question #1

Which ion will increase if an ACID is dissolved in the water? How will the

concentration of the other ion respond?

A.) H3O+ will increase. OH- will increase.

B.) H3O+ will increase. OH- will decrease.

C.) H3O+ will decrease. OH- will increase.

D.) H3O+ will decrease. OH- will decrease.

Analysis Level Question #2

Hydronium & Hydroxide Rule

When a substance dissolves in water and causes the H3O+ concentration to increase, then the OH- concentration will decrease. This signals that the

substance is an ACID.

When a substance dissolves in water and causes the OH-concentration to increase, then the H3O+

concentration will decrease. This signals that the substance is a BASE.

ACIDS = [H3O+] > [OH-]

BASES = [OH-] > [H3O+]

Using Kw to Calculate Ion Concentrations

NOTE : H+ = H3O+ Let’s look at the 19-1 Practice Problems in your

practice packet. We’ll solve numbers 1 and 2 together.

1. What is the concentration of OH- ions in saturated limewater if [H3O+] = 3.98x10-13M? Is the limewater acidic, basic, or neutral?

2. What is the concentration of H3O+ ions in a wheat flour and water solution is [OH-] = 1.0x10-8M? Is wheat flour and water acidic, basic, or neutral?

19-1 Practice Problems #1 What is the concentration of OH- ions in

saturated limewater if [H3O+] = 3.98x10-13M? Is the limewater acidic, basic, or neutral? If the problem involves OH- and H3O+ ions, then

you’ll use the kw equation to solve.

[H3O+][OH-]=1.ox10-14

[3.98x10-13][OH-]=1.0x10-14

[OH-]=0.0251 M The OH- value is larger so limewater is a

base.▪ H3O+ = 3.98x10-13 vs OH-=0.0251

19-1 Practice Problems #2 What is the concentration of H3O+ ions in a

wheat flour and water solution is [OH-] = 1.0x10-8M? Is wheat flour and water acidic, basic, or neutral? If the problem involves OH- and H3O+ ions, then

you’ll use the kw equation to solve.

[H3O+][OH-]=1.ox10-14

[H3O+][1.0x10-8]=1.ox10-14

[H3O+]=1.0x10-6 M

The H3O+ value is larger so wheat flour is acidic.

What is the concentration OH- ions in a potato and water solution if [H3O+] = 1.6x10-6M? Are potatoes and water

acidic, basic, or neutral?A.) [OH-] = 1.6x10-20; basic

B.) [OH-] = 1.6x10-20; acidic

C.) [OH-] = 6.3x10-9; basic

D.) [OH-] = 6.3x10-9; acidic

Application Level Question #3

What is the concentration OH- ions in a potato and water solution if [H3O+] = 1.6x10-6M? Are potatoes and water

acidic, basic, or neutral?A.) [OH-] = 1.6x10-20; basic

B.) [OH-] = 1.6x10-20; acidic

C.) [OH-] = 6.3x10-9; basic

D.) [OH-] = 6.3x10-9; acidic

Application Level Question #3

SET UP:[1.6x10-6][OH-] = 1.0x10-14

What is the concentration of H3O+ ions in 0.1M ammonia if [OH-] = 1.26x10-3M? Is ammonia acidic,

basic, or neutral?A.) [H3O+] = 7.94x10-12; acidic

B.) [H3O+] = 7.94x10-12; basic

C.) [H3O+] = 1.26x10-17; acidic

D.) [H3O+] = 1.26x10-17; basic

Application Level Question #4

What is the concentration of H3O+ ions in 0.1M ammonia if [OH-] = 1.26x10-3M? Is ammonia acidic,

basic, or neutral?A.) [H3O+] = 7.94x10-12; acidic

B.) [H3O+] = 7.94x10-12; basic

C.) [H3O+] = 1.26x10-17; acidic

D.) [H3O+] = 1.26x10-17; basic

Application Level Question #4

SET UP:[H3O+][1.26x10-3] = 1.0x10-14

Individual AssignmentPracticing Kw Calculations

Complete 5-10 of the 19-1 Practice Problems before class tomorrow.

pH Concept

Acidic and basic are two extremes that describe a chemical property chemicals. Mixing acids and bases can cancel out or neutralize their extreme effects. A substance that is neither acidic nor basic is neutral.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

pH Scale

The pH scale is logarithmic. As a result, each whole pH value below 7

is ten times more acidic than the next higher value. ▪ For example, pH 4 is ten times more acidic

than pH 5 and 100 times (10 times 10) more acidic than pH 6.

The same holds true for pH values above 7, each of which is ten times more alkaline (another way to say basic) than the next lower whole value. ▪ For example, pH 10 is ten times more alkaline

than pH 9 and 100 times (10 times 10) more alkaline than pH 8.

pH = O 10,000,000x

pH = 1 1,000,000x

pH = 2 100,000x

pH = 3 10,000x

pH = 4 1000x

pH = 5 100x

pH = 6 10x

pH = 7 NEUTRAL

pH = 8 10x

pH = 9 100x

pH = 10 1000x

pH = 11 10,000x

pH = 12 100,000x

pH = 13 1,000,000x

pH = 14 10,000,000x

Calculating pH

pH = -log[H+] OR pH = -log[H3O+]

pH is defined as the negative logarithm of the hydrogen ion (a synonym for hydronium ion) concentration.

We learned to calculate these concentrations using Kw. Therefore, a pH problem might give me the [H+] concentration directly OR indirectly via [OH-].

Calculating pH Examples

Let’s look at the 19-1 Practice Problems in your practice packet again.

11. Analysis of a sample of maple syrup reveals the concentration of OH- ions is 5.0x10-8 M. What is the pH of this syrup? Is it acidic, basic, or neutral?

12. In a sample of bananas and water, it is found that [H3O+] = 2.51x10-5 M. What is the corresponding pH value, and are the bananas and water acidic, basic, or neutral?

19-1 Practice Problems #11

11. Analysis of a sample of maple syrup reveals the concentration of OH- ions is 5.0x10-8 M. What is the pH of this syrup? Is it acidic, basic, or neutral?

Shoot! You need [H+] to calculation pH, but the problem is giving me [OH-] instead.

Use the kw equation to get [H+] from [OH-]. [H+][5.0x10-8]=1.0x10-14

[H+]=2.0x10-7

Then, use the pH equation. pH= -log[H+] pH=-log[2.0x10-7] pH = 6.7; Maple syrup is slightly acidic since 6.7 is

less than 7.

19-1 Practice Problems #12

12.In a sample of bananas and water, it is found that [H3O+] = 2.51x10-5 M. What is the corresponding pH value, and are the bananas and water acidic, basic, or neutral?

pH= -log[H+] pH=-log[2.51x10-5] pH = 4.6; Bananas are acidic since 4.6 is less than 7.

Additional Example of pH

A solution of HCl is prepared by dissolving 1.2 g of HCl in water to make 350 mL of solution. Determine the pH of this solution.

You’ll need the concentration of the [H+] to calculate the pH. Get this by calculating the MOLARITY of the HCl solution. Molarity = 0.094M pH=-log[0.094] pH=1.03

Individual PracticeCalculating pH Values

Complete #13-#20 of the 19-1 Practice Problems before class tomorrow.

pH’s sister - pOH

pOH = -log[OH-]pH + pOH = 14

Example 16.6 (pg. 575) Calculate the pH and pOH for each of the

following solutions.▪ 1.0x10-3M OH-▪ 1.0 M H+

NOTE: There are multiple ways to calculate these answers.

Individual PracticeCalculating pOH and pH

Examples from page 590-591 in textbook.

32.Calculate the pH corresponding to each of the pOH values listed.

a. pOH = 4.32b. pOH = 8.90c. pOH = 1.81d. pOH = 13.1

Solve the four problems here, and we’ll use the remotes to check your work.

a. Calculate the pH if pOH= 4.32.

Application Level Question # 5pg 590 #32 a

b. Calculate the pH if pOH = 8.90.

Application Level Question #6pg 590 #32 b

c. Calculate the pH if the pOH = 1.81.

Application Level Question #7pg 590 #32c

d. Calculate the pH if the pOH = 13.1.

Application Level Question #8pg 590 #32 d

Individual PracticeMore Calculating pOH, pH, & ions

Solve the following. (pg 590 #33) For each [H+] or [OH-] listed, calculate the

concentration of the other ion, the pH, and the pOH of the solution.a. [H+] = 1.00x10-7 Mb. [OH-] = 4.39x10-5 Mc. [H+] = 4.29x10-11 Md. [OH-] = 7.36x10-2 M

We’ll check answers with the remote in a moment.

What is the pH when the [H+] = 1.00x10-7 M?

(pg.590 #33 a)

Application Level Question #9pg.590 #33 a

What is the pH is the [OH-] = 4.39x10-5M?

(pg. 590 #33 b)

Application Level Question #10pg. 590 #33 b

What is the pOH when the [H+] = 4.29x10-11M?

(pg. 590 #33 c)

Application Level Question #11pg. 590 #33 c

What is the pOH if the [OH-] = 7.26x10-2M?

(pg. 590 #33 d)

Application Level Question #12Pg. 590 #33 d

Suggested Individual Practice

Page 589-591 7-12 (skip 11c) 22 a and c 23 a and c 24 b 25 b 30-31 34-35